Before feeding expandable polymer beads to moulds in which light weight articles such as cups are made, the polymer beads are heated so as to pre-expand them. Conventionally they are then temporarily held in a hopper prior to being fed to the moulds for final expansion.
The beads (which are usually polystyrene) contain a hydrocarbon blowing agent which reaches its boiling point during the time that the beads are within the pre-expansion chamber. The blowing agent vaporises and applies pressure within the heat softened bead walls to expand the beads in what has become known in the art as the pre-expansion stage.
Various pre-expansion methods are known. Steam pre-expansion on a continuous basis or on a batch basis is known but both these methods have the singular disadvantage of requiring a maturity period of at least four hours in order to stabilise cellular pressure within the beads prior to using them for moulding. This system uses direct contact between the steam heating medium and the expandable beads. Because of the maturity period that is required, a steam system requires large hoppers in which the beads can be accommodated while maturing.
An alternative pre-expansion system is that known as dry pre-expansion. In this system the expandable beads are not in direct contact with the heating medium but are pre-expanded by indirect contact with the heating medium through the use of heated cylinder walls. The advantage of this system is that the beads do not need to be matured prior to moulding. Systems of this type are described in U.S. Pat. Nos. 4,060,354 and 3,577,360. The systems described in these two patents use an enclosed heated cylinder in which the beads remain for a fixed time period. In these systems the temperature of the heating medium and the mass of the `shot` of beads charged into the pre-expansion chamber is uniform. The fixed time period during which pre-expansion takes place cannot maintain accurate control of final density. The major reason why the systems of these two prior patents fail to produce pre-expanded beads of uniform density is that the unexpanded beads are not themselves uniform. Firstly, the amount of blowing agent in the beads can vary from bead to bead due to inconsistencies during manufacture of loss of blowing agents subsequent to manufacture. Secondly, and to a more limited degree, there may be size variations in the polymer beads in a particular container, and thirdly the beads in a particular container may be segregated by size.
Because of the inconsistencies referred to, it is unsatisfactory to dry pre-expanded beads for a fixed period of time. These inconsistencies are such that the finished article may have a mass which is anything up to 20% more or less than is desired.